Heat exchanger coating

ABSTRACT

The invention relates to a heat exchanger, in particular an evaporator for an air-conditioning circuit of a vehicle, said exchanger enabling a heat exchange between a first fluid and a second fluid and having surfaces intended to come into contact with one of said fluids, said surfaces being formed from aluminum and/or from aluminum alloy and coated with an alumina layer and with a layer referred to as a reinforcing layer that reinforces the natural protection of the aluminum, said reinforcing layer comprising a portion of organic substances and a portion of mineral substances, said portion of organic substances comprising at least one polymer and said portion of mineral substances comprising at least one substance capable of reacting with the aluminum in order to form an anticorrosion material.

The present invention relates to a heat exchanger, especially for anair-conditioning fluid circuit of a vehicle, and more particularly to anevaporator, and also to a process for manufacturing such an exchanger.

In the air-conditioning systems of vehicles, an evaporator is swept by aflow of air to be cooled. Due to the low temperature of the surfaces ofthe evaporator exposed to the air flow, the moisture contained in thelatter tends to be deposited on these surfaces. The water thus depositedreduces the flow area of the air and prevents direct contact between thelatter and the metal surfaces of the evaporator, which is detrimental tothe heat exchange capacity. Furthermore, soiling adheres to the wetsurfaces, promoting microbial proliferation and the production ofunpleasant odors. The presence of drops of water also causes a corrosionof the surfaces of the exchanger leading to the degradation thereof andthe embrittlement thereof.

In order to overcome these drawbacks, coatings are known that containsubstances intended to form, after drying, on the surfaces of theevaporator, an adherent layer that has hydrophilic, antimicrobial andanticorrosive properties. These coatings are generally deposited inseveral steps, in particular a prior step of surface conversion intendedto enable a good attachment of said layer.

Also known, especially from EP 2 045 559 and WO 200310038471, aretreatment liquids for covering such surfaces that are deposited in asingle step directly after the brazing step. On the other hand, theseliquids form, after drying, on said surfaces, an adherent layer that hasonly film-forming, hydrophilic and antimicrobial properties without realanticorrosive properties.

In order to overcome these drawbacks, there is a need for a coating forthe surfaces of an evaporator that are intended to come into contactwith a medium to be cooled which has anticorrosive properties, evenhydrophilic and antimicrobial properties, and which is deposited in asingle step.

For this purpose, the invention relates to a heat exchanger, inparticular an evaporator for an air-conditioning circuit of a vehicle,said exchanger enabling a heat exchange between a first fluid and asecond fluid and having surfaces intended to come into contact with oneof said fluids, said surfaces being formed from aluminum and/or fromaluminum alloy and coated with an alumina layer and with a layerreferred to as a reinforcing layer that reinforces the naturalprotection of the aluminum, said reinforcing layer comprising a portionof organic substances and a portion of mineral substances, said portionof organic substances comprising at least one polymer and said portionof mineral substances comprising at least one substance capable ofreacting with the aluminum in order to form an anticorrosion material.

In other words, the exchangers are firstly protected by their naturalalumina layer. The coating applied thereto additionally makes itpossible to re-establish protection when the surfaces of the exchangerare subjected to attacks and especially corrosion pitting, by reactingwith the aluminum of the surfaces with which it is then brought intocontact. Furthermore, without alleging to be a precise application forthe phenomena in question, the polymer material(s) used contribute tothe stabilization of the coating on the surfaces, despite the absence ofpretreatment. Said reinforcing coating or layer makes it possible toreinforce the natural protection of the aluminum, especially byregenerating the alumina layer. This coating makes it possible toimprove the resistance of the exchanger to corrosion.

According to various embodiments of the invention, which could be takentogether or separately:

-   -   said alumina layer is itself coated with said reinforcing layer,    -   said reinforcing layer is coated without prior preparation of        the surface, of activation or pickling type,    -   said substance(s) capable of reacting with the aluminum make it        possible to fill in surface irregularities of said surfaces        and/or of the alumina layer thereof,    -   said substance(s) capable of reacting with the aluminum are at        least one chromium-based substance,    -   said chromium-based substance(s) may be trivalent chromium        salts,    -   said portion of organic substances and/or said portion of        mineral substances provide said reinforcing layer with        hydrophilic properties and/or antimicrobial properties,    -   said polymer of the portion of organic substances comprises a        polymer containing free hydroxyl functions, such as polyvinyl        alcohol,    -   said portion of organic substances comprises a binder,    -   said binder is of organic acid, alcohol or amine type and may        react with the hydroxyl functions of said polymer,    -   said portion of organic substances is between 50% and 80%,        preferably between 55% and 65%, by weight,    -   said portion of mineral substances is between 20% and 50%,        preferably between 35% and 45%, by weight,    -   said reinforcing layer additionally comprises at least one        corrosion inhibitor, said corrosion inhibitor(s) reform the        alumina layer in the event of attack,    -   said corrosion inhibitor(s) are at least one titanium-based        substance, said titanium-based substance(s) are titanium salts,    -   said reinforcing layer comprises at least one antimicrobial        substance, said antimicrobial substances may be organic        substances such as bronopol, carbendazim, isothiazolinone and/or        at least one zinc-based substance, such as zinc pyrithione,    -   said reinforcing layer has a surface density of between 0.5 and        1.5 g/m², preferably of between 0.8 and 1.2 g/m²,    -   said exchanger comprises one face, the surface of which is        covered with a reinforcing layer having a thickness greater than        that covering the opposite face,    -   said face covered with a greater thickness is the face exposed        to the external air flow.

The invention also relates to a process for manufacturing an exchanger,in particular an evaporator for an air-conditioning circuit of avehicle, process wherein said exchanger is formed with surfaces intendedto come into contact with one of the fluids to be cooled, said surfacesbeing formed from aluminum and/or from aluminum alloy and coated with analumina layer, and said alumina layer is covered with a coating thatreinforces the natural protection of the aluminum, said coatingcomprising a portion of organic substances and a portion of mineralsubstances, said portion of organic substances comprising at least onepolymer and said portion of mineral substances comprising at least onesubstance capable of reacting with the aluminum in order to form ananticorrosion material.

In other words, according to the process of the invention, the coatingmay be applied directly on the surfaces of the exchanger without a priorsurface treatment and/or preparation step. In yet other words, a singleapplication of coating makes it possible to provide the surfaces of theexchanger with protection against corrosion.

According to various embodiments of the invention, which could be takentogether or separately:

-   -   said surfaces are covered after a step of brazing the exchanger,    -   said coating is applied to said surfaces by spraying and/or        blowing,    -   the exchanger, after having covered its surface with coating, is        blown, before drying, so as to adjust the thickness of coating        present on two of the opposite faces of the exchanger,    -   said coating is dried,    -   said coating is dried during a step of heating between 130° C.        and 180° C., preferably at 150° C.,    -   said heating step lasts from 1 to 10 min; preferably it lasts 5        min,    -   said coating is applied so that the amount of coating applied to        the outer surface of said exchanger is between 15 and 20 ml/m²,    -   the pH of said coating is chosen from between 1.5 and 5,        preferably between 2 and 3.5,    -   said coating comprises a reduced portion of fluoride, of less        than 1000 ppm.

The invention will be better understood, and other objectives, details,characteristics and advantages thereof will become more clearlyapparent, in the course of the detailed explanatory description whichfollows, of at least one embodiment of the invention given by way ofpurely illustrative and nonlimiting example, with reference to theappended FIG. 1 which is a view of a portion of a heat exchangeraccording to the invention.

FIG. 1 represents, in a general manner, a portion of a heat exchangerenabling a heat exchange between a first fluid and a second fluid. Itmay be, for example, an evaporator for a vehicle air-conditioningcircuit. In this case, one of the two fluids is a flow of air to becooled and/or the other fluid a refrigerant. In particular, it is inthis example a brazed evaporator made of aluminum or of aluminum alloy.

Said exchanger is formed here of a bundle 10 comprising ducts 12 forcirculation of the refrigerant. Said ducts 12 are formed, for example,of tubes, connected to one or some collectors, and/or of pairs ofstacked plates, communicating with one another. Between the tubes and/orpairs of plates, it is possible to provide disruptors 14, especiallyseparators of serpentine shape. These separators 14 disrupt thecirculation of the air flow and increase the exchange area. The heatexchange between the air flow and the refrigerant is thus improved.These separators 14 are in contact with the tubes and/or the pairs ofplates, especially via the apices of their bends.

Said exchanger has surfaces intended to come into contact with one ofsaid fluids, especially the flow of air to be cooled, in particular thewalls of said tubes, pairs of plates and/or separators,

According to the invention, said surfaces are formed from aluminumand/or from aluminum alloy. They are coated with an alumina layer andwith a layer referred to as a reinforcing layer that reinforces thenatural protection of the aluminum. This reinforcing layer comprises aportion of organic substances comprising at least one polymer and aportion of mineral substances comprising at least one substance capableof reacting with the aluminum in order to form an anticorrosionmaterial. Said reinforcing layer advantageously constitutes a coating ofsaid alumina layer.

The presence of the mineral substances capable of reacting with thealuminum makes it possible to compensate for attacks on the aluminalayer, or even to regenerate said alumina layer. A protective layer isthus reformed and the consequences of corrosion attacks of the surfacesof the exchanger are limited. By reacting with the aluminum, saidsubstances could also make it possible to fill in surface irregularitiesof said surfaces and/or of the alumina layer thereof.

The organic substances and/or the mineral substances in the coatingcould additionally provide the surfaces of the exchanger withhydrophilic and/or antimicrobial properties. Thus, the drops of waterflatten out in the form of a film which is easier to drain and avoidsthe spraying of water caused by the removal of drops of water,

Said substances are advantageously substances known for fixing oxygenmolecules. Said substances are preferably chromium-based substances, inparticular trivalent chromium salts.

This mechanism may be improved by being in an acid medium, especially ata pH of between 1.5 and 5, preferably between 2 and 3.5. This pH may beobtained by the addition of acids, in particular organic acids thatcould furthermore be binders capable of reacting with the polymer(s) ofthe portion of organic substances. Said polymer comprises, for example,a hydroxylated polymer of vinyl alcohol type. The mechanism is alsoimproved if the coating comprises a small portion of fluoride, of lessthan 1000 ppm.

The anticorrosion properties provided by the substances capable ofreacting with the aluminum may be reinforced by the addition of one ormore corrosion inhibitors to said reinforcing layer. These may betitanium-based substances, especially titanium salts. These corrosioninhibitors may be substances that fix oxygen.

Likewise, for improving the antimicrobial properties, said reinforcinglayer may comprise one or more antimicrobial substances. Thesesubstances may be selected from bronopol, carbendazim, isothiazolinoneand zinc-based substances, such as zinc pyrithione. In the case of acoating having a pH between 1.5 and 5, the antimicrobial substance(s)must withstand the acid media.

Such a heat exchanger may be obtained, for example, by the process ofthe invention.

In a first step, said exchanger is formed with surfaces that areintended to come into contact with one of the fluids to be cooled andsaid surfaces are formed from aluminum and/or from aluminum alloy andcoated with an alumina layer. Preferably, the exchanger is obtained by abrazing step.

In a second step, said alumina layer is covered with a coating thatreinforces the natural protection of the aluminum. Said coatingcomprises a portion of organic substances comprising at least onepolymer and a portion of mineral substances comprising at least onesubstance capable of reacting with the aluminum in order to form ananticorrosion material. According to the invention, the coating isapplied directly after the brazing step without an additional step, suchas a surface treatment of activation or pickling type, or thepreparation for a conventional immersion process—closing off theinternal circuit, inserting into equipment for immersion in a bath.

This step makes it possible to form a layer that regenerates and/or thatreinforces the natural protection of the aluminum, advantageously byregenerating the alumina layer.

As described above, the pH of the coating is preferably between 1.5 and5 and in an optimized manner between 2 and 3.5 in order to obtain a goodregeneration of the alumina layer and a good protection of the surfacesof the exchanger by the reinforcing layer.

The coating is generally applied to the exchanger in a uniform manner,for example by spraying and/or by blowing. Preferably, the amountapplied is between 15 and 20 ml/m².

The covering step may be followed by a step in which the exchanger isblown. This step makes it possible to adjust the thickness of coatingpresent on two of the opposite faces of the exchanger. Generally, thethickness of the coating on the face opposite the spraying and blowingis slightly greater. Preferably, this face is the face exposed to theair flow since this overthickness leads to the corrosion resistance ofthe exchanger being improved. Advantageously, this step takes placebefore the drying step.

These steps are followed by a step in which the coating is dried. Thisdrying step especially enables the formation of the reinforcing layer.Indeed, this step enables the polymerization of the polymer(s) presentin the portion of organic substances of the coating. The polymerizationmay be reinforced by the crosslinking of the polymers, in particular ifcrosslinking agents are added to the coating. The crosslinking agentsmay be of organic acid, epoxy or acrylic type, selected to react withthe free hydroxyl functions of the polymer.

The drying and the polymerization may also be reinforced in anadvantageous manner if said coating is dried during a step of heatingbetween 130° C. and 180° C., preferably at 150° C. Said heating steplasts from 1 to 10 min: preferably it lasts 5 min.

An example of a reinforcing layer that has made it possible to obtain anexchanger having satisfactory hydrophilic, antibacterial andanticorrosion qualities comprises 60% of organic substances and 40% ofmineral substances, including 30% of chromium-based substances. Thepolymer included in the portion of organic substances is especially avinyl alcohol polymer. This layer has a surface density of between 0.8and 1.2 g/m² and an overthickness of 20% on the face exposed to the airflow obtained for example by the blowing step described above.

Comparative tests were carried out between such an exchanger and anexchanger from the prior art.

It was thus shown that such an exchanger had equivalent resultsregarding its hydrophilic, antimicrobial and anti-odor qualities,relative to an exchanger from the prior art known for these samequalities.

In addition, the corrosion resistance is considerably improved owing tothe presence of the reinforcing layer. Corrosion tests made it possibleto demonstrate that:

-   -   the signs of corrosion attack of the aluminum surface were        retarded by more than 60 days on an exchanger according to the        invention;

the depth of the corrosion pitting on the surface of an exchangeraccording to the invention was stable after 90 days in contact with acorrosive solution in a corrosion chamber, whereas the depth of thecorrosion pitting on the surface of an exchanger from the prior ail wasincreased;

-   -   the failure of the exchanger according to the invention is        obtained around 50 days after that of the exchanger from the        prior art.

It should be noted that embodiment variants are of course possible andthat the present invention is not limited to an evaporator for vehicleair conditioning. In particular, it is possible to extend the presentinvention to other types of heat exchanger, and to other fields.

1. A heat exchanger for an air-conditioning circuit of a vehicle, saidexchanger enabling a heat exchange between a first fluid and a secondfluid and comprising: surfaces intended to come into contact with one ofsaid fluids, said surfaces being formed from aluminum and/or fromaluminum alloy and coated with an alumina layer and with a reinforcinglayer that reinforces the natural protection of the aluminum, saidreinforcing layer comprising a portion of organic substances includingat least one polymer and a portion of mineral substances comprising atleast one substance capable of reacting with the aluminum in order toform an anticorrosion material.
 2. The exchanger as claimed in claim 1,wherein said substance(s) capable of reacting with the aluminum allowfor regeneration of the alumina.
 3. The exchanger as claimed in claim 1,wherein said substance(s) capable of reacting with the aluminum allowfor filling in surface irregularities of said surfaces and/or of thealumina layer thereof.
 4. The exchanger as claimed in claim 1, whereinsaid substance(s) capable of reacting with the aluminum are at least onechromium-based or fluoride-based substance.
 5. The exchanger as claimedin claim 1, wherein said portion of organic substances and/or saidportion of mineral substances provide said reinforcing layer withhydrophilic properties and/or antimicrobial properties.
 6. The exchangeras claimed in claim 5, wherein said portion of organic substances isbetween 50% and 80%.
 7. The exchanger as claimed in claim 6, whereinsaid portion of mineral substances is between 20% and 50%.
 8. Theexchanger as claimed in claim 1, wherein said reinforcing layeradditionally comprises at least one corrosion inhibitor.
 9. Theexchanger as claimed in claim 1, wherein said reinforcing layeradditionally comprises at least one antimicrobial substance.
 10. Theexchanger as claimed in claim 1, wherein said reinforcing layer has asurface density of between 0.5 and 1.5 g/m².
 11. The exchanger asclaimed in claim 1, wherein said exchanger comprises one face, thesurface of which is covered with a reinforcing layer having a thicknessgreater than that covering the opposite face.
 12. A process formanufacturing an exchanger for an air-conditioning circuit of a vehicle,forming the exchanger with surfaces intended to come into contact withone of the fluids to be cooled, said surfaces being formed from aluminumand/or from aluminum alloy and coated with an alumina layer, whereinsaid alumina layer is covered with a coating that reinforces the naturalprotection of the aluminum, said coating comprising a portion, oforganic substances comprising at least one polymer and a portion ofmineral substances comprising at least one substance capable of reactingwith the aluminum in order to form an anticorrosion material.
 13. Theprocess as claimed in claim 12, wherein said coating is dried.
 14. Theprocess as claimed in claim 12, wherein the pH of said coating is chosenfrom between 1.5 and
 5. 15. The process as claimed in claim 12, whereinthe coating comprises a reduced portion of fluoride of less than 1000ppm.
 16. The process as claimed in claim 12, wherein said coating isapplied so that the amount of coating applied to the outer surface ofsaid exchanger is between 15 and 20 ml/m².
 17. The heat exchanger asclaimed in claim 1, wherein the heat exchanger is an evaporator.
 18. Theprocess as claimed in claim 12, wherein the heat exchanger is anevaporator.